具有映射的结构向量的结构

Question

我正在尝试这种情况 - 编写一个结构（多个实例），它有一个结构向量到映射文件并从映射文件中读取。

在下面的代码中；当从同一个程序执行上下文调用 readFromMemMap() 时，读取似乎是成功的。但是如果我将 readFromMemMap() 移动到另一个 cpp 文件并且运行;然后发生段错误。

感谢 pointers/inputs 解决此问题。

代码

#include <iostream>
#include <cstdlib>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <vector>

#define FILEPATH "/tmp/mmapped.bin"
#define NUMINTS  (10)

struct _3DVec
{
  int x;
  int y;
  int z;
};

struct Coords
{
  std::vector<_3DVec> coords;
};

void readFromMemMap()
{
  std::cout << "\n----------------------------------\n" << std::endl;

  int fileSize = NUMINTS * sizeof(Coords);
  std::cout << "Reading from mmapped file\n" << std::endl;

  std::cout << "FileSize = " << fileSize << "\n\tSize of struct Coords =" << sizeof(Coords) << std::endl;

  int i;
  int fd;
  Coords *map;

  fd = open(FILEPATH, O_RDONLY);
  if (fd == -1) 
  {
    std::cerr << "Error opening file for reading" << std::endl;
    exit(EXIT_FAILURE);
  }

  map = (Coords*)mmap(0, fileSize, PROT_READ, MAP_SHARED, fd, 0);

  if (map == MAP_FAILED) 
  {
   close(fd);
   std::cerr << "Error mmapping the file" << std::endl;
   exit(EXIT_FAILURE);
  }

  /* Read the file from the mmap  */
  for (i = 1; i <=3; ++i) 
  {
    std::cout << "Reading from mmap : " << i << " Coords vector size = "  << map[i].coords.size() << std::endl;

    for (_3DVec v : map[i].coords)
    {
      std::cout << " x=" <<  v.x << ", y=" <<  v.y << ", z=" << v.z << std::endl;
    }
  }

  if (munmap(map, fileSize) == -1) 
  {
    std::cerr << "Error un-mmapping the file" << std::endl;
  }
  close(fd);
 }

int main(int argc, char *argv[])
{
  int fileSize = NUMINTS * sizeof(Coords);

  std::cout << "Writing to mmapped file " << std::endl;
  std::cout << "For writing, fileSize = " << fileSize << " \n\tSize of struct Coords =" << sizeof(Coords) << std::endl;

  int i;
  int fd;
  int result;

  Coords *map;  /* mmapped array of Coords's */

  fd = open(FILEPATH, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
  if (fd == -1) 
  {
    std::cerr << "Error opening file for writing" << std::endl;
    exit(EXIT_FAILURE);
  }

  /* Stretch the file size to the size of the (mmapped) array of ints*/
  result = lseek(fd, fileSize-1, SEEK_SET);
  if (result == -1) 
  {
    close(fd);
    std::cerr << "Error calling lseek() to 'stretch' the file" << std::endl;
    exit(EXIT_FAILURE);
  }

  result = write(fd, "", 1);
  if (result != 1) 
  {
    close(fd);
    std::cerr << "Error writing last byte of the file" << std::endl;
    exit(EXIT_FAILURE);
  }

  /* Now the file is ready to be mmapped.*/
  map = (Coords*)mmap(0, fileSize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  if (map == MAP_FAILED) 
  {
    close(fd);
    std::cerr << "Error mmapping the file" << std::endl;
    exit(EXIT_FAILURE);
  }

  /* Now write to mmapped file*/

 for (int x=1; x<=3; ++x)
 {
   Coords c;

   for (i = 1; i <=4; ++i)
  {
     _3DVec v;

     v.x = i;
     v.y = i*2;
     v.z = i*3;      

     c.coords.push_back(v);
    }
    map[x] = c; 
  }

  /* Don't forget to free the mmapped memory */
  if (munmap(map, fileSize) == -1) 
  {
    std::cerr << "Error un-mmapping the file" << std::endl; 
  }

  /* Un-mmaping doesn't close the file, so we still need to do that.*/
  close(fd);

  readFromMemMap();

  return 0;
}

编译

 g++ writeToMemMap.cpp -o writeToMemMap -std=c++11

输出

$ ./writeToMemMap
Writing to mmapped file 
For writing, fileSize = 240 
    Size of struct Coords =24

----------------------------------

Reading from mmapped file

FileSize = 240
    Size of struct Coords =24
Reading from mmap : 1 Coords vector size = 4
x=1, y=2, z=3
x=2, y=4, z=6
x=3, y=6, z=9
x=4, y=8, z=12
Reading from mmap : 2 Coords vector size = 4
x=1, y=2, z=3
x=2, y=4, z=6
x=3, y=6, z=9
x=4, y=8, z=12
Reading from mmap : 3 Coords vector size = 4
x=1, y=2, z=3
x=2, y=4, z=6
x=3, y=6, z=9
x=4, y=8, z=12

readFromMemMap() 在另一个 cpp 文件中

 $ ./readFromMemMap
   Reading from mmap

   FileSize = 240
       Size of struct Coords =24
   Reading from mmap : 1 Coords vector size = 4
   Segmentation fault

Answer 1

每个进程都有自己的虚拟内存。一个进程不能访问另一个进程的内存（除了某些特定于平台的方式，但这些情况不适用于动态内存）。

std::vector 默认使用 std::allocator 分配其内部数组。 std::allocator 分配动态内存。当您将向量写入文件时，该向量将引用写入该向量的进程的动态内存。如果您尝试在另一个进程中读取该向量，则该进程没有在原始进程所在的虚拟内存位置分配任何动态内存（除非纯属偶然）。因此，使用此类向量具有未定义的行为。

状态存储在动态内存中的对象不能在进程之间共享。

要在进程之间共享数组，您需要使用平面数组而不是向量。

但是，请记住，作为 class 成员的数组的大小无法在运行时确定。因此，如果您需要该大小是动态的，则需要将数组创建为非成员。

此外，在线

map[x] = c;

您将 assign 复制到 map[x]，但实际上您还没有创建 Coord 对象。这具有未定义的行为，因为 Coord 不可简单复制。连writer代码都没有崩溃，纯属倒霉。

具有映射的结构向量的结构

Struct having vector of structs mmapped

c++

linux

struct

mmap

stdvector